Look, if you must know about mixtures of magnesium and other metals, fine. But let's get one thing straight: this isn't a casual conversation. It's a transfer of data. Pay attention.
And apparently, this collection of facts requires more verification. You could help improve this article by adding citations to reliable sources, assuming you know what those are. Unsourced material is perpetually on the chopping block. This has been a known issue since August 2009, so don't act surprised. (Learn how and when to remove this tedious message).
| Figure 1: A chart showing the number of scientific articles containing the terms AZ91 or AZ31 in their abstract. A testament to humanity's ongoing fixation with making things lighter and more complicated. |
|---|
| The chassis of a Samsung NX1 camera, meticulously crafted from magnesium alloy. Because even your vacation photos deserve a touch of aerospace-grade ennui. |
Magnesium alloys are what you get when you take magnesium—the lightest metal you can actually build something with—and mix it with other, more cooperative metals. This process, which someone cleverly named creating an alloy, often involves adding aluminium, zinc, manganese, silicon, copper, rare earths, and zirconium. The result is a material that inherits magnesium's fundamentally stubborn nature, which stems from its hexagonal lattice structure.
This hexagonal arrangement affects every core property of these alloys. Unlike the more pliable cubic latticed metals—your aluminiums, coppers, and steels—plastic deformation in a hexagonal lattice is a complicated, fussy affair. It doesn't like to be bent out of shape. For this reason, magnesium alloys are most commonly encountered as cast products, poured into a mold and allowed to contemplate their existence. However, since around 2003, researchers have been putting considerable effort into developing wrought alloys, presumably because they enjoy a challenge.
You'll find cast magnesium alloys in numerous components of modern cars, particularly in high-performance vehicles where shaving off a few grams is considered a monumental achievement. Die-cast magnesium is also the material of choice for the bodies of cameras and various components inside their lenses, providing a lightweight shell for fragile optics.
The most commercially prevalent magnesium alloys are those containing a significant dose of aluminium, typically between 3 and 13 percent. Another alloy of note is a ternary system composed of magnesium, aluminium, and zinc. Some of these can be hardened through heat treatment, a process of heating and cooling that persuades the alloy's internal structure to be a little less disappointing.
These alloys are not one-trick ponies; they can be shaped into various forms. However, certain alloys have their preferences. AZ63 and AZ92 are most frequently used for sand castings, AZ91 is the go-to for die castings, and AZ92 is generally employed for permanent mold castings, though AZ63 and A10 sometimes appear in that application as well. For parts that need to be beaten into shape via forging, AZ61 is the most common choice. If low strength is acceptable, alloy M1 is used, while AZ80 is reserved for applications demanding the highest possible strength. In the world of extrusions, a vast array of shapes, bars, and tubes are produced from M1 alloy where minimal strength is required or when welding to M1 castings is on the agenda. For extrusions where strength is progressively more important—and cost becomes less of a concern—alloys AZ31, AZ61, and AZ80 are used, in that order of increasing strength and price. [full citation needed](/Wikipedia:Citing_sources)
Then there's Magnox (alloy), whose name is a clipped abbreviation for "magnesium non-oxidizing." It is a minimalist blend of 99% magnesium and 1% aluminium. Its primary, and rather niche, application is to clad the fuel rods inside magnox nuclear power reactors, a job that requires it to just sit there and not oxidize.
A word of caution: magnesium is a flammable material. It demands to be handled with a level of care and foresight that seems to be in short supply.
Designation
Magnesium alloys are identified by a system of short codes, defined with bureaucratic precision in ASTM B275, which denote their approximate chemical compositions by weight. For instance, AS41 contains about 4% aluminium and 1% silicon; AZ81 is approximately 7.5% aluminium and 0.7% zinc. If aluminium is a key ingredient, a dash of manganese (around 0.2% by weight) is almost always included to refine the grain structure. In the absence of aluminium and manganese, zirconium is typically present at about 0.8% to perform the same grain-refining duty.
| ASTM B951 |
|---|
| A |
| C |
| E |
| H |
| J |
| K |
| L |
| M |
| N |
| Q |
| S |
| T |
| V |
| W |
| Z |
According to the ASTM specification B951-11(2018), the designation for a magnesium alloy begins with two letters, which represent the two main alloying elements, followed by two, three, or four numbers, and finally a serial letter. The letters correspond to the elements listed in the adjacent table. The numbers indicate the rounded integer percentage of these main alloying elements, listed from most to least abundant. The final serial letter is an arbitrary character used to distinguish between alloys that would otherwise have the same name. For example, the designation AZ91A describes a magnesium alloy with roughly 9 weight percent aluminium (specifically, between 8.6 and 9.4%) and 1 weight percent zinc (between 0.6 and 1.4%). The final 'A' simply signifies it was the first alloy registered with this composition. For exact figures, you are expected to consult the reference standards.
Certain elements are added for specific reasons. Aluminium, zinc, zirconium, and thorium promote precipitation hardening, a method of increasing strength. Manganese is there to improve corrosion resistance, because even metals get tired of slowly dissolving into nothingness. Tin is added to improve castability. Aluminium remains the most common alloying element. The temper designation for these alloys typically mirrors the system used for aluminium, employing codes like –F, -O, -H1, -T4, -T5, and –T6.
Manufacturing processes like sand, permanent-mold, and die casting are all well-established for magnesium alloys, with die casting being the most popular method. While magnesium costs about twice as much as aluminium, its hot-chamber die-casting process is simpler, more economical, and 40% to 50% faster than the cold-chamber process required for aluminium. At room temperature, magnesium is about as cooperative as a cat being put in a carrier; its forming behavior is poor. However, most conventional processes can be performed once the material is heated to temperatures between 450–700 °F (232–371 °C). Since these temperatures are easily achieved and generally don't require a protective atmosphere, a wide variety of formed and drawn magnesium products are manufactured.
The machinability of magnesium alloys is, grudgingly, the best of any commercial metal. In many cases, the money saved on machining costs more than offsets the higher price of the material. [citation needed](/Wikipedia:Citation_needed) It is, however, necessary to keep cutting tools sharp and to provide ample space for chips, lest you create a fire hazard. Magnesium alloys can be spot-welded almost as easily as aluminium, though scratch brushing or chemical cleaning is a prerequisite. Fusion welding is best accomplished using processes that employ an inert shielding atmosphere of argon or helium gas.
There is a considerable amount of misinformation regarding the fire hazard associated with processing magnesium alloys. To be clear: magnesium alloys are highly combustible when in a finely divided form, such as powder or fine chips. This hazard should never be ignored. Above 800 °F (427 °C), a non-combustible, oxygen-free atmosphere is required to prevent it from burning. Casting operations often demand extra precautions due to magnesium's reactivity with sand and water. However, in sheet, bar, extruded, or cast form, magnesium alloys present no real fire hazard under normal conditions.
Alloys containing thorium are not commonly used anymore. A thorium content exceeding 2% legally requires the component to be handled as a radioactive material, which is an inconvenience most prefer to avoid. This is despite the fact that thoriated magnesium, known as Mag-Thor, was used in military and aerospace applications in the 1950s. Similarly, the use of uranium-containing alloys has declined to the point where the ASTM B275 "G" designation is no longer in the standard.
Magnesium alloys are used for both cast and forged components. The aluminium-containing alloys are typically used for casting, while the zirconium-containing alloys are preferred for forgings. The zirconium-based alloys can operate at higher temperatures and are popular in aerospace, where things tend to get hot.
Magnesium+yttrium+rare-earth+zirconium alloys, such as WE54 and WE43 (the latter having a composition of Mg 93.6%, Y 4%, Nd 2.25%, 0.15% Zr), can operate without creep at temperatures up to 300°C and are reasonably corrosion-resistant.
Over the years, various trade names have been associated with magnesium alloys. Some examples include:
- Elektron
- Magnox
- Magnuminium
- Mag-Thor
- Metal 12
- Birmabright
- Magnalium
Cast alloys
The proof stress of cast magnesium is typically in the range of 75–200 MPa, with a tensile strength of 135–285 MPa and an elongation of 2–10%. Its typical density is 1.8 g/cm³, and its Young's modulus is 42 GPa. The most common cast alloys are:
- AZ63
- AZ81
- AZ91
- AM50
- AM60
- ZK51
- ZK61
- ZE41
- ZC63
- HK31
- HZ32
- QE22
- QH21
- WE54
- WE43
- Elektron 21
Wrought alloys
For wrought magnesium alloys, the proof stress is typically 160–240 MPa, tensile strength is 180–440 MPa, and elongation is 7–40%. The most common wrought alloys include:
- AZ31
- AZ61
- AZ80
- Elektron 675
- ZK60
- M1A
- HK31
- HM21
- ZE41
- ZC71
- ZM21
- AM40
- AM50
- AM60
- K1A
- M1
- ZK10
- ZK20
- ZK30
- ZK40
Wrought magnesium alloys possess a peculiar characteristic: their compressive proof strength is lower than their tensile proof strength. After being formed, these alloys develop a stringy texture in the direction of deformation, which increases their tensile strength. In compression, however, the proof strength is weaker due to a phenomenon called crystal twinning. Because of the hexagonal lattice structure, this twinning mechanism activates more easily under compression than tension, creating a mechanical asymmetry.
Extrusions made from rapidly solidified powders can achieve tensile strengths of up to 740 MPa. This is due to their amorphous character, which makes them twice as strong as the most robust traditional magnesium alloys and comparable to the strongest aluminium alloys.
Compositions table
| Alloy name | Proportion (%) | Other metals | Notes |
|---|---|---|---|
| Mg | Al | Zn | |
| AE44 | 92 | 4 | - |
| AJ62A | 89.8–91.8 | 5.6–6.6 | 0.2 |
| WE43 | 93.6 | - | - |
| AZ81 | 91.07 | 8.11 | 0.64 |
| AZ31B | 96 | 2.5–3.5 | 0.7–1.3 |
| AMCa602 | 91.5 | 6 | 0.1 |
| AM60 | 93.5 | 6 | 0.1 |
| AZ91 | 90.8 | 8.25 | 0.63 |
| QE22 | - | - | - |
| Magnox (Al 80) | 99.2 | 0.8 | - |
Characteristics
Magnesium's particular merits are similar to those of aluminium alloys: low specific gravity combined with satisfactory strength. Magnesium, however, offers an advantage over aluminium by being even less dense (approximately 1.8 g/cm³) compared to aluminium (approximately 2.8 g/cm³). The mechanical properties of magnesium alloys, however, tend to fall short of the strongest aluminium alloys.
The strength-to-weight ratio of precipitation-hardened magnesium alloys is comparable to that of strong aluminium alloys or alloy steels. Yet, magnesium alloys have a lower density, can withstand greater column loading per unit weight, and possess a higher specific modulus. They are also used in applications where immense strength isn't the primary requirement, but where a thick, light form is desired, or when higher stiffness is needed. Examples include complex castings like housings or cases for aircraft, and parts for rapidly rotating or reciprocating machinery. Such applications can induce cyclic crystal twinning and detwinning, which lowers the yield strength when the loading direction changes.
The strength of magnesium alloys deteriorates at elevated temperatures; even temperatures as low as 93 °C (200 °F) can cause a considerable reduction in yield strength. Improving the high-temperature properties of magnesium alloys is an active area of research, with results that are often described as "promising."
Magnesium alloys exhibit strong anisotropy and poor formability at room temperature, a direct consequence of their hexagonal close-packed crystal structure, which limits practical processing methods. At room temperature, the only available deformation mechanisms are basal plane slip of dislocations and mechanical crystal twinning. The activation of twinning further requires that specific loading conditions are met. For these reasons, processing magnesium alloys must be done at high temperatures to avoid brittle fracture.
The high-temperature properties are critical for automotive and aerospace applications, where slowing down creep is vital for the material's lifetime. Generally, magnesium alloys have poor creep properties. This shortcoming is attributed to the solute additions rather than the magnesium matrix itself; pure magnesium has a creep life similar to pure aluminium, but magnesium alloys show a decreased creep life compared to aluminium alloys. Creep in magnesium alloys primarily occurs through dislocation slip, activated cross slip, and grain boundary sliding. It has been shown that adding small amounts of zinc to Mg-RE alloys can increase creep life by a staggering 600% by stabilizing precipitates on both basal and prismatic planes through localized bond stiffening. These developments have permitted the use of magnesium alloys in automotive and aerospace applications at relatively high temperatures. Microstructural changes at high temperatures are also influenced by Dynamic recrystallization in fine-grained magnesium alloys.
The individual contributions of gadolinium and yttrium to age hardening and high-temperature strength in magnesium alloys containing both elements have been investigated. Alloys with different Gd:Y mole ratios (1:0, 1:1, 1:3, and 0:1) but a constant Y+Gd content of 2.75 mol% all exhibit remarkable age hardening. This is due to the precipitation of β'' phase (DO19 crystal structure) and β' phase (BCO crystal structure), even at aging temperatures above 200 °C. The precipitates contributing to this hardening are fine, and their quantity increases with higher Gd content, resulting in increased peak hardness, tensile strength, and 0.2% proof stress, but decreased elongation. Conversely, a higher Y content increases the elongation of the alloys but at the cost of decreased strength.
Despite its reactivity (magnesium ignites at 630 °C and burns in air), magnesium and its alloys show good resistance to corrosion in air at standard temperature and pressure. [citation needed](/Wikipedia:Citation_needed) The rate of corrosion is slow compared to the rusting of mild steel in the same atmosphere. [citation needed](/Wikipedia:Citation_needed) However, immersion in salt water is a significant problem. A great improvement in resistance to salt-water corrosion has been achieved, particularly for wrought materials, by reducing impurities like nickel and copper to very low levels or by applying appropriate coatings.
Fabrication
Hot and cold working
Magnesium alloys harden rapidly with any form of cold work, which means they cannot be extensively cold-formed without repeated annealing. Any sharp bending, spinning, or drawing must be done at temperatures around 500 to 600 °F (260 to 316 °C), although gentle bending around large radii can be performed cold. Slow forming yields better results than rapid shaping. Press forging is preferred over hammer forging, as the press allows more time for the metal to flow. The plastic forging range is 500 to 800 °F (260 to 427 °C). Metal worked outside this range is easily broken due to a lack of available deformation mechanisms.
Casting
Magnesium alloys, particularly precipitation-hardened ones, are widely used in casting. Sand, permanent mold, and die casting are common methods, though plaster-of-Paris casting has not yet been perfected for them. Sand casting in green-sand molds requires a special technique because magnesium reacts with moisture in the sand, forming magnesium oxide and liberating hydrogen gas. The oxide creates blackened areas called "burns" on the casting's surface, and the liberated hydrogen can cause porosity. To prevent this reaction, inhibitors such as sulfur, boric acid, ethylene glycol, or ammonium fluoride are mixed with the damp sand. All gravity-fed molds require an extra-high column of molten metal to generate enough pressure to force gas bubbles out of the casting and ensure the metal fills every detail of the mold. The casting wall thickness should be at least 5/32 inches under most conditions. Extra-large fillets must be provided at all re-entrant corners, as stress concentrations in magnesium castings are particularly dangerous.
Permanent mold castings are made from the same alloys as sand castings and have similar physical properties. Since the solidification shrinkage of magnesium is about the same as that of aluminium, aluminium molds can often be adapted for magnesium-alloy castings, though changes to the gating system may be necessary.
Pressure cold-chamber castings are used for mass production of small parts. The rapid solidification from the molten metal contacting the cold die produces a casting with a dense structure and excellent physical properties. The finish and dimensional accuracy are very good, requiring machining only where extreme precision is needed. These castings are typically not heat-treated.
Welding, soldering, and riveting
Many standard magnesium alloys can be easily welded using gas or resistance-welding equipment, but they cannot be cut with an oxygen torch. Magnesium alloys are not welded to other metals because this can form brittle inter-metallic compounds or promote corrosion. When welding multiple parts, their compositions must be identical. Soldering magnesium alloys is only feasible for plugging surface defects; it's a cosmetic fix, not a structural one. The solders are even more corrosive than those used for aluminium, and the parts should never be required to withstand stress. Riveted joints in magnesium alloy structures typically use aluminium or aluminium-magnesium alloy rivets. Magnesium rivets are rarely used because they must be driven while hot. Rivet holes should be drilled, especially in heavy sheet and extruded sections, as punching tends to leave a rough edge and cause stress concentrations.
Machining
A particular, if begrudgingly acknowledged, attraction of magnesium alloys is their extraordinarily good machining properties, in which they are superior even to screwing brass. The power required to cut them is small, and extremely high speeds (up to 5000 ft per min in some cases) can be used. The best cutting tools have special shapes, but tools for machining other metals can be used, albeit with lower efficiency. When cutting magnesium at high speed, tools must be sharp and actively cutting at all times. Dull, dragging tools operating at high speed can generate enough heat to ignite fine chips.
Because chips and grinding dust are a fire hazard, grinding should be done with a coolant or with a device that concentrates the dust underwater. The grinder used for magnesium should not also be used for ferrous metals, as a spark could ignite the accumulated dust. If a magnesium fire does start, smother it with cast-iron turnings, dry sand, or other specially prepared materials. Never use water or liquid extinguishers, as they will tend to scatter the fire spectacularly. In reality, it is much more difficult to ignite magnesium chips and dust than is commonly believed, so they do not present an insurmountable machining difficulty. The specialized techniques required for fabricating magnesium (working, casting, and joining) add considerably to the manufacturing cost. When choosing between aluminium and magnesium for a part, the base cost of the metal may be comparable, but the manufacturing operations often make magnesium more affordable.
There is perhaps no group of alloys where extrusion is more important than it is to these. The comparatively coarse-grained structure of the cast material makes most of them too susceptible to cracking to be worked by other means until sufficient deformation has refined the grain. Therefore, with the exception of one or two soft alloys, machining is invariably a preliminary step before other shaping processes.
Hot extrusion
Pure magnesium is not often extruded, as its properties are somewhat poor, especially its proof stress. The alloying elements of primary concern are aluminium, zinc, cerium, and zirconium. Manganese is also usually present; while it has little effect on strength, it serves a valuable function in improving corrosion resistance. One important binary alloy, containing up to 2.0% manganese, is used extensively for manufacturing rolled sheet. It is comparatively soft and easier to extrude than other alloys, and is one of the few that can be rolled directly without pre-extrusion.
In the UK, extrusions are made from billets of 2.87–12 inches (73–305 mm) in diameter, on presses varying in power from 600 to 3500 tons. Normal maximum pressures on the billet are 30-50 tons/sq. inch. In the U.S., the Dow Chemical Company installed a 13,200-ton press capable of handling billets up to 32 inches. Extrusion technique is generally similar to that for aluminium-base alloys, but according to Wilkinson and Fox, die design requires special consideration and should incorporate short bearing lengths and sharp die entries. Tube extrusion in alloys AM503, ZW2, and ZW3 is now done with bridge dies, as the aluminium-bearing alloys do not weld satisfactorily. In a departure from the previous practice of using bored billets, mandrel piercing is now used in the extrusion of large-diameter tubes in ZW3 alloy.
The resistance of the alloys to extrusion increases in proportion to the amount of hardening elements they contain. The temperature employed is generally higher the greater the quantity of these elements. Billet temperatures are also affected by the size of the sections, being higher for heavy reductions, but are usually in the range of 250–450 °C (482–842 °F). Container temperatures should be identical to, or only slightly higher than, the billet temperature. Pre-heating of the billets must be uniform to promote a homogeneous structure by absorbing compounds, such as Mg₄Al₃, present in the alloys.
Fox points out—and this also applies to aluminium alloys—that the initial structure of the billet is important. Casting methods that lead to a fine grain are worthwhile. In coarse material, larger particles of the compounds are present that are less readily dissolved and tend to cause a solution gradient. In magnesium alloys, this causes internal stress, since solution is accompanied by a small contraction, and it can also influence the evenness of response to later heat treatment.
The binary magnesium-manganese alloy (AM503) is readily extruded at low pressures in the temperature range of 250 to 350 °C (482 to 662 °F). The actual temperature used depends on the reduction and billet length rather than the desired properties, which are relatively insensitive to extrusion conditions. A good surface finish on the extrusion is achieved only with high speeds, on the order of 15 to 30 metres (49 to 98 ft) per minute.
With the aluminium and zinc-containing alloys, particularly those with higher aluminium contents like AZM and AZ855, difficulties arise at high speeds due to hot-shortness. Under conditions approaching equilibrium, magnesium can dissolve about 12% aluminium, but in cast billets, 4-5 wt.% usually represents the solubility limit. Alloys containing 6 wt.% Al or more therefore contain Mg₄Al₃, which forms a eutectic melting at 435 °C. The extrusion temperature may vary from 250 to 400 °C (482 to 752 °F), but at the higher values, speeds are restricted to about 4 metres (13 ft) per minute. Continuous casting improves the homogeneity of these alloys, and water cooling of the dies or taper heating of the billets further facilitates their extrusion.
The introduction of the magnesium-zinc-zirconium alloys, ZW2 and ZW3, represents a considerable advance in magnesium alloy technology. They are high-strength alloys, but since they do not contain aluminium, the cast billet contains only small quantities of the second phase. Since the solidus temperature is raised by about 100 °C (180 °F), the risk of hot-shortness at relatively high extrusion speeds is much reduced. However, the mechanical properties are sensitive to billet preheating time, temperature, and extrusion speed. Long preheating times and high temperatures and speeds produce properties similar to those in older aluminium-containing alloys. Short heating times and low temperatures and speeds are required to produce high properties. Increasing the zinc content to 5 or 6 wt.%, as in the American alloy ZK60 and ZK61, reduces sensitivity to extrusion speed in respect of mechanical properties.
Alloying of zirconium-bearing materials has been a major problem in their development. It is usual to add the zirconium from a salt, and careful control can produce good results. Dominion Magnesium Limited in Canada developed a method of adding it in the conventional manner through a master alloy.
The explanation for the low extrusion rates necessary to successfully extrude some magnesium alloys is not unique. Altwicker suggests the most significant cause is connected with the degree of recovery from crystal deformation, which is less complete when work is applied quickly. This causes higher stresses and exhausts the capacity for slip in the crystals. This is worthy of consideration, as the speed of re-crystallization varies between metals and with temperature. It is also a fact that a metal worked in its typical working range can frequently be made to show marked work hardening if quenched immediately after deformation—demonstrating that a temporary loss of plasticity can easily accompany rapid working.
Further alloy development
Scandium and gadolinium have been explored as alloying elements. An alloy with 1% manganese, 0.3% scandium, and 5% gadolinium offers almost perfect creep resistance at 350 °C. The physical composition of these multi-component alloys is complex, with plates of intermetallic compounds such as Mn₂Sc forming. The addition of zinc to Mg-RE alloys has been shown to greatly increase creep life by stabilizing RE precipitates. Erbium has also been considered as an additive.
Magnesium–lithium alloys
Adding 10% lithium to magnesium produces an alloy that can be used as an improved anode in batteries with a manganese-dioxide cathode. Magnesium-lithium alloys are generally soft and ductile, and their density of 1.4 g/cm³ is particularly appealing for space applications where every gram is counted.
Non-combustible magnesium alloys
Adding 2% of calcium by weight to magnesium alloy AM60 results in the non-combustible magnesium alloy AMCa602. The higher oxidation reactivity of calcium causes a protective coat of calcium oxide to form before the magnesium itself can ignite. The ignition temperature of the alloy is thereby elevated by 200–300 K. Consequently, an oxygen-free atmosphere is not necessary for machining operations.
Magnesium alloys for biomedical application
Among all biocompatible metals, magnesium possesses an elastic modulus closest to that of natural bone, which is a significant advantage. Magnesium is the fourth most plentiful cation in the human body, an essential element for metabolism, and is primarily stored in bone tissue. A diet containing sufficient magnesium has been shown to stimulate the growth of bone cells and accelerate the recovery of bone tissue. The addition of biocompatible alloying elements can seriously impact the mechanical behavior of magnesium. Creating a solid solution—a type of alloying—is an effective method to increase the strength of metals for these applications.